1. Field of the Invention
This invention relates to a heat-resistant Ni-base single crystal alloy. More specifically, it relates to a heat-resistant Ni-base single crystal alloy suitable as a material for blades and vanes of gas turbines or high-pressure turbines of jet engines.
2. Description of the Prior Art
The limit temperature for using materials, from which turbine blades and vanes in gas turbines or jet engines are made, is an important factor determining the thermal efficiency of the engines. In particular, the turbine blades require high creep strength because they undergo high centrifugal forces during high-speed rotation.
Conventional Ni-base heat resistant alloys are composed of Al and Ti as elements constituting the gamma prime phase, Co, W, Mo, Ta and Nb as solution strengthening elements, Cr as a corrosion-resistant element and C, B and Zr as grain boundary strengthening elements, and retain high strength at high temperatures by precipitating the gamma prime particles in the Ni matrix phase (gamma phase). With increasing amounts of the gamma prime phase and the solution strengthening elements, the alloys increase in strength, but decrease in ductility.
In order to remove this defect, a directional solidification technique was developed in which a crystal grain boundary perpendicular to the stress axis which is a crack generating source in polycrystals at high temperatures is erased and columnar crystals composed of a bundle of elongate crystals are made. Thereafter, nickel-base alloys in which the columnar crystal grain boundary is strengthened by adding Hf was developed and became commercially available as PWA-1422.
Later, it became known from studies on the heat-treatment of columnar crystals that when the amount of the coarse gamma prime re-dissolved is increased by raising the solution heat-treatment temperature, the amount of fine gamma prime particles precipitated by aging treatment increases, and the creep strength of the resulting alloy increases. But it also became known that since the alloys contain grain boundary strengthening elements which reduce the incipient melting temperature, it is not possible to increase the solution heat-treatment temperature to a temperature high enough to obtain a complete solution of the coarse gamma prime, and there is a limit to the creep strength improvement.
With the foregoing background, single crystal alloys began to be developed which contain grain boundary strengthening elements (Hf, C, B, Zr) in amounts corresponding only to impurities and have an incipient melting temperature higher than the temperature at which all the coarse gamma prime particles dissolve into the gamma phase, and segregation of the elements can be sufficiently removed.
Alloy 444 and Alloy 454 (made by United Technology Inc.; their compositions will be given hereinafter) were developed as the first single crystal alloys. The creep strength of these alloys shows a great increase and their temperature capability increases by about 30.degree. C. from that of the columnar crystal alloy (PWA-1422).
Later, CMSX-2 and CMSX-3 (made by Cannon Muskegon Corporation; their compositions are given hereinafter) were developed. The strengths of these alloys are almost the same as that of Alloy 454. To increase strength, it is necessary to increase the amounts of Co and W. Since, however, precipitation of a detrimental phase occurs, the amount of Co is limited to not more than 7% by weight, and the amount of W, to not more than 8.4% by weight.